Commit f670c47f authored by simonpj@microsoft.com's avatar simonpj@microsoft.com

Fix Trac #3406 (albeit not very satisfactorily): scoped type variables

The issue here is this:

  type ItemColID a b = Int  -- Discards a,b

  get :: ItemColID a b -> a -> ItemColID a b
  get (x :: ItemColID a b) = x :: ItemColID a b

The pattern signature for 'x' doesn't actually rigidly bind a,b.
This crashed GHC 6.10 with a 'readFilledBox' panic.  Now we fail
with an erroe message

With the new outside-in algorithm we'll be able to accept this program.
parent 738f7078
......@@ -39,6 +39,7 @@ import TcIface
import TcType
import {- Kind parts of -} Type
import Var
import VarSet
import Coercion
import TyCon
import Class
......@@ -48,6 +49,7 @@ import PrelNames
import TysWiredIn
import BasicTypes
import SrcLoc
import Util
import UniqSupply
import Outputable
import FastString
......@@ -891,6 +893,16 @@ tcPatSig ctxt sig res_ty
-- Check that pat_ty is rigid
; checkTc (isRigidTy res_ty) (wobblyPatSig sig_tvs)
-- Check that all newly-in-scope tyvars are in fact
-- constrained by the pattern. This catches tiresome
-- cases like
-- type T a = Int
-- f :: Int -> Int
-- f (x :: T a) = ...
-- Here 'a' doesn't get a binding. Sigh
; let bad_tvs = filterOut (`elemVarSet` exactTyVarsOfType sig_ty) sig_tvs
; checkTc (null bad_tvs) (badPatSigTvs sig_ty bad_tvs)
-- Now match the pattern signature against res_ty
-- For convenience, and uniform-looking error messages
-- we do the matching by allocating meta type variables,
......@@ -1041,6 +1053,15 @@ wobblyPatSig sig_tvs
<+> pprQuotedList sig_tvs)
2 (ptext (sLit "unless the pattern has a rigid type context"))
badPatSigTvs :: TcType -> [TyVar] -> SDoc
badPatSigTvs sig_ty bad_tvs
= vcat [ fsep [ptext (sLit "The type variable") <> plural bad_tvs,
quotes (pprWithCommas ppr bad_tvs),
ptext (sLit "should be bound by the pattern signature") <+> quotes (ppr sig_ty),
ptext (sLit "but are actually discarded by a type synonym") ]
, ptext (sLit "To fix this, expand the type synonym")
, ptext (sLit "[Note: I hope to lift this restriction in due course]") ]
scopedNonVar :: Name -> Type -> SDoc
scopedNonVar n ty
= vcat [sep [ptext (sLit "The scoped type variable") <+> quotes (ppr n),
......
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